The present invention relates to contact lenses to correct vision and more particularly to bifocal contact lenses.
There have been efforts to make bifocal contact lenses which provide both for reading and for more remote viewing, for eyes having limited muscular range of focus. Such efforts generally involved, in the past, attempts to permit the contact lens wearer to shift focus by looking through a smaller part of the lens having a greater magnifying effect. The magnifying effect was obtained by using a material of higher refractive index than the rest of the lens or by using a radius of external curvature of a part of the lens different from the radius of curvature of the remainder of the lens.
In U.S. Pat. No. 3,726,587 entitled "Bifocal Corneal Contact Lens And Method Of Making Same", a bifocal corneal contact lens consists of a curved disk-like segment having one index of refraction which is fused into an indentation in the surface of the contact lens body having a different index of fefraction. The fused segment has both inside and outside curvatures which are formed to correct the wearer's vision. In U.S. Pat. No. Re. 25,286 entitled "Bifocal Corneal Contact Lens", one zone of the lens is adapted to correct for distance vision and the other zone for reading vision. The distance vision zone is a small zone in the center which is one-half of the size of the pupil and has a slightly smaller radius of curvature than the remainder of the lens. A fused bifocal contact lens is shown in U.S. Pat. No. 3,472,581 entitled "Fused Multifocal Corneal Contact Lenses". The two materials that are employed have different refractive indexes and specifically the anterior has a high refractive index and the posterior portion has a lower refractive index, so that the distance portion of the lens is located at the center of the lens.
It is practically impossible to provide, in a contact lens, for a bifocal effect dependent upon shift of the line of vision (as in the case of bifocal glasses), because the cornea moves with the pupil in looking up and down. In order to present a differently made portion of the lens to the pupil when looking away from the center, the lens would have to slide over the cornea in response to eyelid friction. However, a contact lens is adapted to remain in one centered location on the cornea and cannot slide over the cornea in response to desired changes in focus.